The employees were charging a furnace for an alloy melt. Employee #1 had just placed the final two pigs on the lip of the furnace door and had backed his lift truck at an angle to a distance of 15 feet from the furnace. Employee #3 had previously pushed seven pigs into the furnace. As he pushed the final two pigs into the furnace, there were two separate explosions inside the furnace. The first explosion caused a wave of molten metal to flow over the open furnace sill outward to a distance of 40 feet. The second explosion caused a fan shaped flame that extended out 80 feet from the furnace. The flame touched the left front of Employee #1's lift truck and completely engulfed Employee #3's lift truck (located 30 feet directly in front of the furnace). Employee #2 was standing behind Employee #1's lift truck. All three employees were burned. Employee #3 died of his injuries. Water pockets inside the pigs apparently caused the explosions.

 Source: osha.gov